In a small saucepan, add in the milk and butter and bring it to a gentle simmer over medium heat.
Dump the bread flour into the milk mixture and remove the saucepan from heat. Stir vigorously with a wooden spoon for 1-2 minutes.
Next, add in the tapioca flour, sugar, vanilla, matcha, salt, and stir until it forms a thick paste. It will look slightly gelatinous and lumpy.
Let the mixture cool down and then drizzle in the beaten egg a little bit at at time.
Stir the beaten egg into the tapioca mixture and continue to add in the egg until the mixture becomes a lava-like consistency (similar to making a choux, the mixture should drop from a spatula in the shape of a "v").
As with making cream puffs, be careful not to add too much egg or the batter will be too runny and will not form properly. The consistency of the dough should be thick enough to hold its shape when piped.
Add in black sesame seeds (if using) and mix to combine.
Transfer the batter to a pastry bag (no piping tip is necessary), and cut the tip off.
Pipe rounds about 1" in diameter onto a parchment-lined baking sheet, spacing them apart.
Have a little dish of clean water ready. Dip your fingertip in the water to gently press down any pointy tops.
Sprinkle with additional black sesame seeds (for garnish) if you like.
Bake at 375°F/191°C for the first 5 minutes, and then reduce to 350°F/177°C for 15-16 minutes, until barely golden in colour.
Remove from the oven and let cool before enjoying.